Documents [1], [2] describe the benefits of overlapping cooperation areas. Simulations are indicating strong performance improvements and as already discussed in document [3], it is an open issue how to schedule the user equipment (UE) to the best suited CA, if several CAs are candidates for that UE.
A scenario is assumed with multiple (shifted) cooperation areas patterns (hereinafter referred to as ‘cover shifts’) that overlap each other, as illustrated in FIG. 1. The cooperation areas within the same cover shifts are non overlapping. In FIG. 1, each gray-scale represents a cover shift. Always three sectors contribute to a cooperation area.
In the example shown in FIG. 1, with five CA-shifts, all possible borders between two adjacent CAs (existing of three cell sites) can be covered by a different sub-band. In FIG. 1, all three contributing sites of a CA are connected with a triangle. The center area is shaded corresponding to the sub-band used by this CA. Since all cells are members of all shifts, the system is a so-called reuse one system.
The principle benefit of the cover shift is shown in FIG. 2. UEs at the border between two non overlapping CAs of the same shift (here: Shift ‘A’) are considered. These three illustrated UEs can be served as CA-edge-UEs by one of both CAs in cover shift ‘A’. Further, they can be served as CA-center-UEs in a CA of cover shift ‘B’. Each cover shift uses its own part of the system's resources, i.e. a certain frequency block, timeslot, etc. These segregations are creating an orthogonality and subsequently there is no interference between different cover shifts.
Each cover shift must have enough resources to serve all of its assigned UEs. The UEs in turn should be connected to the CA that contains at least their strongest, for example, three eNBs regarding the wideband radio channel connectivity according to the ideas presented in documents [1], [2], and [4]. However, it is noted here that three is only an example and any other suitable number of strongest eNBs could be used. In the following examples, the orthogonalization between cover shifts is described by using different sub-bands from a given system bandwidth. I.e. in FIG. 2, the cover shift A and the cover shift B are using different non-overlapping sub-bands of the system bandwidth.
As described in documents [1], [2] and [3], a clustering is assumed, where three adjacent cell sites cooperate, each cell site has three cells (120° sectors), which leads to 9 cell cooperation areas. The UE is conducting measurements and does a ranking of the measured cells according to the measured performance indicators (e.g. RSRP) and for cooperation it selects the 3 strongest cells. The N=3 strongest cells can concern to one, two, or three different cell sites. Due to an overlapping CA clustering as shown in FIG. 1, each cell site is a member of six different CAs. Two adjacent cell sites belong to two different CAs and three adjacent cell sites define exactly one CA.
Thus, depending of the distribution of its three strongest cells, each UE can be assigned to one, two, or even three different CAs. FIG. 3 shows an example of such cases. According to FIG. 3, CAs existing of nine cells, organized in three adjacent cell sites area assumed. Hence, the concept of shifted CAs results in six different shifts, marked as triangles with edges at the three cooperating cell sites in FIG. 3. There, three dark colored adjacent cells as assumed to be the strongest ones of an UE. As shown in FIG. 3, a set of three adjacent cells can be contained in one, two or all six different CA shifts.
The problem to be solved is an assignment and scheduling problem.
Namely, how to do the assignment of UEs and the sub-bands to CAs such that:                1. each CA has enough bandwidth to serve the assigned UEs, and        2. each UE is allocated to the optimum CA.        
Due to the overlapping CAs, there are more possibilities for each UE to be assigned to. If additionally, the different CAs use different sub-bands, the assignment and scheduling should not be done by two independent processes.
In conventional solutions, each cell (i.e., each nodeB) has its own frequency resources that are not reused in adjacent cells, like in GSM. In reuse one scenarios like UMTS, each cell uses the full spectrum, but suffers from inter cell interference especially at the cell borders.
Fixed clustered cooperation areas (CAs) avoid interference between cooperating cells, but between two adjacent CAs, the problem is still present, i.e. an UE at CA borders suffers on inter CA interference.
The only possibility is to choose the best suited resource blocks for each UE, where best suited means that it suffers not too strong from inter cell interference. Thus, scheduling can only utilize frequency selective fading to mitigate the influence of inter cell interference, after the UEs being assigned to the nodeBs. This yields to poor performance since often strong interferers in adjacent CAs degrade the Signal to Interference Ratio (SIR).